In order to reduce the coupling-in of interference signals, e.g. in RF circuits (radiofrequency circuits), signal-carrying paths are embodied as so-called “balanced” conducted signal lines. Balanced conducted differential-mode lines mean an increased circuitry outlay; however, they are less susceptible to common-mode interference and are therefore preferred for carrying signals in RF circuits—e.g. in mobile communication devices. Instead of the term “balanced” conducted signal lines, the term “symmetrically” conducted signal line is also used synonymously. “Unbalanced” conducted signal lines are correspondingly “asymmetrically” conducted signal lines.
Antennas of mobile communication devices generally supply a common-mode signal. Therefore, a balun circuit (balun=balanced/unbalanced converter) is necessary in order to convert the common-mode signal into a differential-mode signal for differential-mode lines. Furthermore, a filter circuit is necessary in order to suppress interfering frequencies in frequency ranges—e.g. outside a relevant passband.
Components operating with surface acoustic waves (SAW components, SAW=surface acoustic wave) can provide a good filter characteristic and also an intrinsic balun functionality. However, the technology of SAW components is restricted with regard to the maximum operating frequency and with regard to the bandwidth to be provided. Furthermore, SAW components always require a monocrystalline piezoelectric substrate, which is expensive and intricate to produce.
BAW resonators are better suited to use at higher frequencies and, on account of their manner of production, can be better integrated into external circuit environments based on silicon. While structures of coupled BAW resonators (e.g. in SCF=stacked crystal filter) are suitable, in principle, for forming filters having intrinsic balun functionality, there is the problem that the layer thickness dependence of the resonant frequency is to be taken into consideration and the necessary resonator quality factor should be achieved. SCF structures, in particular, react sensitively to thickness fluctuations such as occur during the deposition of the piezoelectric layers. Such fluctuations then impair the quality factor of the entire filter circuit.
U.S. Pat. No. 7,196,596 B2 describes FBAR filters (thin film bulk acoustic resonator filter) having an unbalanced conducted signal input and having a balanced conducted signal output. Corresponding filter circuits comprise FBAR resonators and also circuits of a balun which are electrically connected thereto. A balun comprises passive circuit components such as e.g. coupled transmission lines or capacitive or inductive elements, wherein the passive circuit components are arranged in metallization layers between dielectric layers. The layer stack composed of metallization layers and dielectric layers in which the balun functionality is integrated is arranged on a substrate alongside the stack containing the electrodes and piezo-layers of the FBAR resonators. The layer stack of the balun can be produced during the same process steps as the layers of the hermetic covering of the FBAR resonators.
Miniaturization of electrical components is still the most important requirement, principally from the mobile radio sector, in order to be able to make terminals even smaller, but principally to be able to equip them with additional functions. In addition to the demand for a small space requirement, it is a matter of concern for the manufacturers of electrical components to obtain a highest possible percentage of functioning modules, i.e. a lowest possible percentage of rejects, during the production process.
Therefore, it is an exemplary object of the present invention to specify a BAW component having balun functionality with a high resonator quality factor and a small space requirement, which component can be produced with a lower reject rate.